This invention relates to pressure sensitive adhesive compositions particularly adapted for use on vinyl substrates. These adhesives comprise polymers of ethylene, vinyl ester, di-2-ethylhexyl maleate or di-n-octyl maleate or the corresponding fumarate, and an unsaturated monocarboxylic acid.
Pressure-sensitive adhesives function to provide instantaneous adhesion when applied under light pressure. They are characterized by having a built-in capacity to secure this adhesion to a surface without activation, such as by treatment with solvents or heat, and also by having sufficient internal strength so that the adhesive material will not rupture before the bond between the adhesive material and the surface ruptures. The capacity to obtain instantaneous adhesion is usually expressed as the amount of "tack" or "tackiness". Ordinarily it is desirable to obtain as much tack as possible without losing a significant amount of internal strength (cohesion). The latter balance of adhesive and cohesive properties has been difficult to obtain in adhesive polymers since monomers conventionally incorporated into the polymers to increase the cohesive strength generally result in a decrease in adhesive tack.
A specific class of pressure sensitive adhesives are used on vinyl substrates such as are utilized in a wide variety of industrial applications including as vinyl roof tops in automobiles, for decorative trim as in vinyl labels, and decals and in specialty types. Unlike common polyolefin films such as polyethylene and polypropylene, these polyvinyl chloride (PVC) films by nature of their process requirements and high glass transition temperature, require formulation with a number of additives. Some of these are migratory components and will affect the long-term performance of a vinyl decal or label. The amounts and types of additives vary among vinyl formulations, therefore, a wide range of vinyl characteristics are available. A typical flexible PVC film formulation contains a number of additives including plasticizers. Plasticizers used include the migratory monomeric types such as phthalate esters (e.g., dioctylphthalate) or the less migratory polymeric plasticizers (e.g., polyesters, epoxidized soybean oils). These applications require stringent requirements on the adhesive composition. In particular, the vinyl substrates contain substantial amounts of these plasticizers which tend to migrate into the adhesive, especially after aging, the migration of which will destroy the adhesive and cause the bond to fail.
Thus, the use of adhesive-coated flexible polyvinylchloride films has traditionally been plagued, to various degrees, by deterioration of properties due to migration of plasticizer out of the PVC film into the adhesive coating. The degree to which this occurs is dependent on the PVC formulation and the adhesive used. In addition to resistance to this plasticizer migration on aging, adhesives for use on vinyl decals must also possess the previously discussed high initial peel values both cohesive and adhesive strength and must inhibit resistance to shrinkage.
Two of the most important factors influencing the real-life performance of an adhesive-coated vinyl decal or label are the plasticizer type and level in a PVC film. These additives which allow use of PVC for flexible applications are a major cause of the deterioration of polymer coatings applied to PVC. As a result of their more migratory character, monomeric plasticizers will have a bigger effect on adhesive properties than will polymer plasticizers. All else being equal, the higher the content of monomeric plasticizer in a vinyl film, the more deterioration experienced by the adhesive-coated decal as a function of time.
To accurately predict the extent of plasticizer migration and its effects on adhesive properties, it is important to address what happens to the plasticizer when it leaves the vinyl film. If the adhesive totally blocks out the plasticizer, the result is an accumulation of plasticizer at the adhesive-vinyl interface. This can result in bond failure at this boundary. If the adhesive is too accepting of the plasticizer, the viscoelastic properties of the adhesive deteriorate resulting in a weak, leggy polymer film. A third possibility involves migration of the plasticizer out of the PVC, through the adhesive mass, to the adhesive-substrate interface. This can retard or destroy bond formation.
The destiny of the plasticizer after the migration process is actually a combination of these three extremes. The degree to which one case predominates is dependent on adhesive, substrate and the PVC backing. The most obvious indication of plasticizer migration related adhesive deterioration is the degradation of adhesive properties as a function of aging on vinyl. Many adhesive systems display a dramatic drop off in peel adhesion and creep resistance during contact with plasticized vinyl; however, through proper polymer design, control over the migration process and the subsequent effect on the pressure sensitive adhesive properties can be achieved.